A desirable encapsulation for a surface mount device is epoxy. Epoxy is well-suited to mass production molding, providing a structurally sound and hermetically-sealed enclosure for the encapsulated components. A surface-mount device is installed by soldering it in place, requiring the application of heat perhaps as high as 200.degree. C. which causes the components of the display to expand. However, if the respective coefficients of thermal expansion (CTE) of the encapsulation, the circuit board, and other components differ markedly, they will expand at proportionately different rates, leading to stress and strain, causing the device to crack and probably fail.
The heat problem for surface mount devices can be dealt with in several ways. First, there is a generally-accepted engineering practice of specifying printed circuit board materials having a low CTE to match the CTE of the copper traces on the circuit board within the device. This will minimize expansion of the circuit board and avoid cracking of the copper traces and other damage to the device. Second, the expansion of the encapsulation can be minimized by employing an opaque epoxy, which has a CTE nearly equal to that of the encapsulated components. The CTE of the epoxy can be further lowered by adding impurities to the epoxy. Thus, when heat is applied, the encapsulation and the components inside will expand at a nearly-equal rate.
For some applications, it would be desirable to provide an LED alphanumeric display in a surface-mount configuration. The active display devices, related logic circuitry, and the interconnecting leads would be placed on a substrate such as a printed circuit board or flex circuit, and the assembly would be encapsulated within a transparent enclosure. Since it is imperative that the encapsulation for a display device be transparent, opaque epoxies and impurities cannot be employed. Nevertheless, it would be desirable to encapsulate the device in epoxy. However, the required clear epoxy has a CTE considerably higher than that of the circuit boards, components, and interconnecting leads of the typical display device. If no provision is made for the high CTE of clear epoxy, the device will likely fail as a result of the heat from soldering.
To avoid failure due to uneven expansion of the various components and the clear epoxy encapsulation of the LED alphanumeric display, the composition of the components and encapsulation should be selected such that the coefficients of thermal expansion (CTE) of those components are equivalent or nearly so.